We designed a polymerase chain reaction method to detect herpes simplex virus (HSV) DNA in spinal fluid from patients with encephalitis. The polymerase chain reaction amplified a 211 base-pair segment of the HSV DNA polymerase gene. Applying this method, we diagnosed HSV type 1 infection in three young children, aged 7 to 13 months, who had atypical forms of the illness. On the basis of magnetic resonance imaging, their disease was diffuse or multifocal in two cases and, in all three, lacked the temporal lobe involvement considered characteristic of HSV encephalitis beyond the neonatal period. Most of the diffuse or multifocal abnormalities detected by magnetic resonance imaging were not apparent by computed tomography. Restriction enzyme analysis of the polymerase chain reaction products from all three patients indicated that their disease was caused by HSV type 1. We conclude that in preschool-age children beyond the neonatal period, the spectrum of HSV encephalitis includes multifocal or diffuse involvement of the brain, which may be detected most efficiently by magnetic resonance imaging. The polymerase chain reaction method has the potential for providing an early diagnosis, but further studies are required to define the sensitivity and specificity of the polymerase chain reaction before it can be used for routine clinical decision making. (J PEDIATR 1995;126:234-41).